The goal of the proposed study is to define new mechanisms regulating the phototransduction and adaptation processes in visual receptors. The major focus is to determine the physiological role of transducin GTPase regulation messenger in the photoresponse. This proposal is based on the recent finding that cGMP phosphodiesterase, specifically its gamma- subunit, can accelerate the GTPase activity of that transducin which activates it. cGMP binding to the non-catalytic sites of phosphodiesterase alpha and beta subunits suppresses the GTPase acceleration. This suggests that cGMP is not only a excitatory messenger in phototransduction but may also regulate the duration of the photoresponse through a feedback mechanism. The first aim of this catalytic cGMP-binding sites are responsible for GTPase regulation and to define the item scale on which cGMP dissociation from these sites causes GTPase acceleration. This will indicate whether the cGMP- dependent suppression of transducin GTPase is a candidate for involvement in recovery rom the single photoresponse or/and photoreceptor adaptation to background illumination. The second aim is to make a quantitative description of the phosphodiesterase activation-inactivation cycle in the presence and absence of cGMP bound to the non-catalytic sites. The obtained parameters will be correlated with the rate of transducin GTPase measured under the same conditions. This correlation will indicate whether phosphodiesterase turnoff is determine only by transducin GTPase or additional mechanisms are involved. The third aim is to determine the epitope on the acceleration. The long-term goal of this part of the work is to look for activity in different G-proteins by their effectors. The experiments proposed are relevant to understanding the intricate feedback controls that regulate photoreceptor activity, controls that may be perturbed in several inherited retinal diseases.